One method of switching an optical signal from one path to another is by means of an optical cross-connect switch. In particular, free-space optical cross-connect switches perform this switching by re-directing an information-bearing collimated laser beam from one route to another as the light traverses a free-space path. This re-direction is accomplished by varying the trajectory on which the collimated laser beam is launched into the free-space path, so that it exits on a different route from the opposite end of the free-space path.
Prior free-space optical cross-connect switches employ schemes for varying the launch trajectory of a collimated laser beam that require the use of a companion pilot beam for each information-bearing beam being launched. Each of these pilot beams is aligned with a specific information-bearing beam and employed to guide that beam onto a photo-receptor which serves as a gateway to the desired exit route. Typically, each photo-receptor is situated at the center of an array of photo-detectors. In operation, a particular information-bearing beam, along with its companion pilot beam, is launched toward a particular photo-receptor. The surrounding photo-detectors sense the pilot beam, and provide feedback to a controller which directs the information-bearing beam directly onto the proper photo-receptor.
While this type of free-space optical cross-connect switch has proved effective, it requires a significant amount of hardware to implement the pilot beam system. In addition, as the pilot beam provides the sole means for insuring proper targeting of the information-bearing beam, misalignment between the pilot and information-bearing beams could result in a failed cross-connection. Another drawback of this type of optical cross-connect switch is the limited ability of the photo-detectors to locate the pilot beam. If a pilot beam is not incident upon one of the photo-detectors, the cross-connect switch is left "blind" as to the location of the pilot beam. This deprives the controller of any feedback to aid in properly targeting the information-bearing beam associated with the pilot beam.